it maybe the ballast, bulb and ballast r not cheap. I would try the problem bulb on the working ballasts to narrow down the problems.
Outer bulb just have to physically check. If its ok it maybe fuse or bad ground.
Never touch the light bulb with bare hands. Clean it with alcohol if you have accidentally touch it to avoid grease on the bulb which would cause hot spot during the operation. Also if the bulb is not securely mounted to the housing, vibration could cause the fillament to break.